A conventional image reading apparatus uses a white color reference member for a shading calibration, and reads a reading side of an original document passing through the white reference member. However, a shadowing phenomenon, in which variation of color density on the other side of the original document is shown and read throw the original document, may occur in the reading operation of the image reading apparatus. In order to reduce the shadowing phenomenon, a gray reference member is provided for some image reading apparatuses.